Solar cells are expected as an electric source of the next generation and more particularly, as a diffusion installation type electric source from the viewpoints that it uses the unexhaustible sun light as its energy source, and it hardly affects the surroundings, it operates efficiently even in the case where it is of a small size, and its maintenance is easy. There have been proposed a number of photoelectromotive force members to be used for solar power generation. And it has been reported that there will be obtained a photoelectric conversion efficiency of more than 20% in the case where a crystal of Si or GaAs is used as its constituent semiconductive layer. However, such photoelectromotive force members are very costly and therefore, the cost of electric power unavoidably becomes higher than that obtained by other electric power generating means, because these devices have not yet come to practical use. On the other hand, as for thin film photoelectromotive force elements such as amorphous silicon pin type, Schottky type and CdS-CdTe heterojunction type, cost reduction of the element itself can be anticipated, but its photoelectric conversion efficiency has not come to exceed 10% on a practical scale, and the cost required for the installation becomes rather high, and accordingly, it is impossible to sufficiently reduce the price of the electric power. Various proposals for increasing the photoelectric conversion efficiency of a thin film photoelectromotive force element have been made to improve the situation.
In one such proposal, for the thin film photoelectromotive force element, one of the constituent n-type and p type thin film semiconductor layers is constituted with a 100 .ANG. or less thick semiconductor thin film containing an impurity and another semiconductor thin film of less than 100 .ANG. in thickness containing said impurity in a different concentration from the above, or a different impurity or layers containing the different impurities being repeatedly laminated.
This proposal will be explained with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view of a pin type A Si:H thin film photoelectromotive force element to which the above proposal has been applied. In FIG. 1, there are shown a metallic substrate 1 such as a stainless steel plate having a polished surface, a plurality of n-type 50 .ANG. A thick A-Si:H film layers 2 formed using PH.sub.3 alternately stacked with a corresponding plurality of 50 .ANG. thick A-Si:H film layers 3 formed not using said PH.sub.3, a 5000 .ANG. thick A-Si:H film layer 4 not containing any impurity, a plurality of p-type 50 .ANG. thick A Si:H film layers 5 formed using B.sub.2 H.sub.6, alternately stacked with a corresponding plurality of a 50 .ANG. thick A-Si:H film layers 6 formed not using said B.sub.2 H.sub.6, a transparent electroconductive layer 7 such as an ITO film and a collector electrode 8, for example, made of silver.
By employing this stacked structure, the short-circuit current (Isc) can be made larger than that of the conventional pin type photoelectromotive force element shown in FIG. 2. This seems to be due to that in the element shown in FIG. 2, the photons absorbed in the n-type thin film semiconductor layer 22 or the p-type thin film semiconductor layer 26 scarcely generate electrons and holes, and on the other hand, in the element shown in FIG. 1, the electrons and holes can be effectively generated in the stacked structure of n-type and p-type thin film semiconductor layers. However, even in the case of the element in FIG. 1, improvements in the open circuit voltage (Voc) and the fill factor (F.F.) are not sufficient, and it is not possible to remarkably increase the photoelectric conversion efficiency. In addition, in the case where the above proposal is applied to a photosensor with reverse bias voltage, it is preferable that the dark current (corresponding to the saturation current of a diode) be as small as possible, since in that case, the S/N ratio may be increased and as a result, the range of the luminous intensity required for a photosensor, may be extended. However, at present the devices have not yet come to sufficiently satisfy the practical demands.